US20090044794A1 - Inducer speed control method for combustion furnace - Google Patents

Abstract

In a multistage combustion furnace having a motor driven inducer blower and a pressure sensing device or set of switches for sensing pressure in the combustion gas flowpath through the furnace, a high firing rate blower speed and low firing rate blower speed are set based on the blower speed setting required for a medium firing rate. Each particular furnace, having its own pressures and combustion gas resistance to flow characteristics, may be provided with a database of inducer blower motor speeds required to achieve predetermined pressures in the combustion gas flowpath for a variety of combustion gas venting systems generating such resistance.

Description

BACKGROUND OF THE INVENTION
• Efficient combustion furnaces for heating, ventilating and air conditioning (HVAC) equipment applications are typically provided with a motor driven so-called ventilating or inducer blower which draws air through the combustion passageways of the furnace heat exchanger to improve the efficiency of the combustion and heat transfer processes and to prolong the life of the furnace. Selection of the proper speeds of the inducer blower drive motor for multistage combustion furnaces, in particular, has been a somewhat nettlesome problem. Combustion furnaces which include electronic controls have been developed wherein the inducer blower motor speed is controlled based on opening and closing of pressure switches which measure the pressures developed by the inducer blower at one or more particular points in the air flowpath.
• Moreover, so-called learning algorithms have been developed which require setting a blower default speed for multistage furnaces for the low firing rate and high firing rate which is the first speed that the inducer motor will be controlled to when a call for the low firing rate or high firing rate is signaled to the furnace controller. The inducer blower then “learns” a speed based on opening and closing of the pressure switches. Still further, typically, a low speed limit is defined in the control system program to avoid the combustion gas control valve closing prematurely. U.S. Pat. Nos. 6,257,870 and 6,377,426 to Hugghins, et al. and assigned to the assignee of the present invention disclose and claim methods for setting inducer blower operating speeds.
• However, for multistage furnaces including, for example, three stage furnaces, it is desirable to maintain the inducer flow pressures above a predetermined setpoint which is particularly important at low firing rates to avoid low combustion gas pressures which could create undesirable combustion characteristics. This occurs because the gas valve output pressure tracks the inducer or system pressures in the aforementioned type of furnace. Still further, it is desirable to simplify the “learning” of the inducer blower motor speeds for respective furnace firing rates in multistage furnaces, including three stage furnaces. In accordance with the present invention, improvements in determining and setting inducer blower speeds and operating pressures have been realized and attendant advantages enjoyed as a result.
SUMMARY OF THE INVENTION
• The present invention provides an improved method of determining proper inducer or ventilating blower speeds for multistage combustion furnaces for HVAC applications
• In accordance with one aspect of the present invention, a method of controlling the inducer or ventilating blower speed and the pressures generated thereby has been developed wherein a control system for the furnace is programmed to provide, initially, a default speed for a medium or intermediate furnace firing rate, for example. A learn routine is provided for the medium firing rate and a multiple of the learned medium firing rate inducer blower speed value is applied to set a blower motor speed for a low firing rate and a high firing rate based on the speed at the medium or intermediate firing rate. These multiples may be based on the realization that there is a substantially linear relationship between properly set inducer blower speed and the flow resistance caused by the venting system connected to the furnace. Thus, the lower inducer blower speed limits and upper speed limits can be defined by this relationship and set as multiples of the learned medium or intermediate firing rate inducer blower speed. Inducer blower speeds determined in this manner allow for proper furnace operation with virtually any type of venting system normally expected to be connected to the furnace.
• In accordance with another aspect of the present invention there is provided a method of controlling a combustion furnace, including a multistage furnace and, particularly, a three-stage furnace wherein the furnace control system learns a medium speed for the inducer blower motor to which is applied a multiple or multiplier (less than one and greater than one) to provide inducer blower speeds for a furnace low firing rate and a high firing rate. The multiplier may be different for different models of furnace, but may be provided to the control system for a particular furnace when it leaves the point of manufacture or at a later time. Accordingly, a learning procedure for the inducer blower motor speed at low firing rates and high firing rates is not required and the so-called target inducer blower speeds for low and high firing rates are actually relatively closer to a learned speed than arbitrarily selected default speeds.
• In accordance with a further aspect of the present invention a method of determining inducer blower speeds is provided wherein one or more inducer blower speeds for particular firing rates of a furnace may be preset and based on respective multiples or multipliers of another selected motor speed and which multiples may be developed through testing the flow resistance of various lengths and configurations of furnace venting systems likely to be applied to respective different furnace models. The multipliers would possibly be different for different furnace models and could be provided to a particular furnace control system as part of a set of control and operating parameters programmed in the control system directly or on a separate information or “personality” module associated with the furnace prior to or after shipment from the point of manufacture.
• Still further, the invention contemplates the provision of a furnace control system and method of operation wherein only a single pressure switch or pressure sensor would be required to properly operate the furnace at different inducer blower speeds.
• Those skilled in the art will further appreciate the above-mentioned advantages and superior features of the invention, together with other important aspects thereof, upon reading the detailed description which follows in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a perspective view, in somewhat schematic form, of a multistage combustion furnace operable in accordance with the method of the present invention;
• FIG. 2 is a schematic diagram of a control system for the furnace illustrated inFIG. 1; and
• FIG. 3 is a flow diagram illustrating certain steps in the method of the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
• In the description which follows like elements are marked through the specification and drawing with the same reference numerals, respectively. The drawing figures are not necessarily to scale and certain elements are shown in schematic or somewhat generalized form in the interest of clarity and conciseness.
• Referring toFIG. 1, there is illustrated a combustion furnace operable by the method of the present invention and generally designated by thenumeral10. Thefurnace10 is illustrated as including a generally rectangularboxlike cabinet12 having an air inlet opening12aadapted to be connected to areturn air duct12b.Cabinet12 also includes a discharge air opening12cfor discharging air through suitable ducting to an enclosedspace13. Airflow to and from thefurnace10 is via suitable ducting and in accordance with the direction of flow indicated by thearrows13aand13b.Combustion furnace10 includes plural side-by-side gaseous fuel firedheat exchangers14, each being provided with a serpentine combustion gas flow passage14a,one shown, and each discharging combustion gases and ventilation air to aplenum14bin a known manner. Additional details of combustion gas furnaces of the general type referred to herein are described in U.S. Pat. No. 5,060,722 to Zdenek, et al. and U.S. Pat. No. 5,309,892 to Lawlor, et al., both of which are assigned to the assignee of the present invention.
• Air circulated to and from thespace13 is propelled throughcabinet12 by a motor drivenair circulation blower15 disposed within the cabinet, as illustrated inFIG. 1. Combustion fuel is delivered to theheat exchangers14 atrespective burners16 which are supplied with fuel, such as natural gas, by way of a manifold17 connected to acontrol valve18 having a suitableelectric controller20 associated therewith. An electric motor driven inducer or ventilatingblower22 is operably associated with theplenum14b for drawing air and combustion gases through theheat exchangers14 in a known manner. Blower22 is controlled by anelectric drive motor23 which includes a suitable speed control system to be described in further detail herein. A temperature sensor or so-calledthermostat24 is disposed in thespace13 and is operably connected to afurnace control system26 disposed at thecabinet12.
• Control system26 may include aninterface28 for use by a user or service technician for setting certain control parameters and observing certain operating conditions offurnace10.Control system26 includes, for example, amicrocontroller30 for receiving signals from thethermostat24 and for controlling operation of theblowers15 and22 and the fuelflow control valve18. Suitable pressure sensors or switches, three shown by way of example, are designated bynumerals32a,32band32cinFIG. 1 and are shown mounted onplenum14bfor sensing the pressure therein. Thepressure sensors32a,32band32cmay be characterized as pressure switches which open and close at fixed or adjustable preset pressures and are suitably disposed within the flow path of the combustion gases and air being circulated by theblower22. Other locations of the sensors orswitches32a,32band32cmay be selected as compared with the location illustrated. Moreover, the switches orsensors32a,32band32cmay be replaced by a single variable pressure sensor which is operable to output signals to thecontrol system26, indicating the pressure associated with the air and combustion products flowstream flowing through theheat exchangers14.
• Combustion products discharged from thefurnace10 are conducted through a vent system orflue pipe25 suitably connected to theblower22 and normally having a length sufficient to conduct combustion gases to the exterior of the structure or building in which thefurnace10 is located. Accordingly, the so-called venting system, including theflue pipe25, may be of various lengths and configurations and may include one or more pipe elements which are curved. Thus, a certain resistance to flow of combustion products would be associated with the configuration of a particular venting system. Accordingly, each furnace design or configuration, including its venting system, would have a set of inducer blower motor speeds corresponding to furnace and venting system flow characteristics and required to produce desired pressures and flow rates through the heat exchanger passages14aandplenum14b.Of course, the pressures that theinducer blower22 is capable of producing in the combustion gas flowpath, including the burner or heat exchanger passages14aand theplenum14b,will vary with the speed of the blower and itsdrive motor23.
• Referring now toFIG. 2, there is illustrated a schematic diagram ofcontrol system26 which includes amicroprocessor30 operable to receive signals from thethermostat24 and thepressure switches32a,32band32c,as well as a limit temperature sensor orswitch33 disposed incabinet12.Microprocessor30 is operable to control the operation ofmotor15aby way of amotor control circuit15b,the operation ofvalve18 and the operation ofinducer blower motor23 by way of its ownspeed control circuit23a.User interface28 may be used to observe certain operating parameters of thecontrol system26 and make selections of such parameters via a suitable useroperable keypad28aand avisual display28b.Still further,microprocessor30 may includecertain memory circuits30aand30bwhich are operable to receive information from a separate circuit of adevice35 which may be releasably connected to themicroprocessor30 and sometimes referred to as a so-called personality module. Thepersonality module35 may be of a type described in co-pending U.S. patent application Ser. No. 11/717,466 filed Mar. 13, 2007, by Robert W. Helt, et al. and assigned to the assignee of the present invention. Still further,thermostat24 includes, for example, temperature andhumidity sensors24aand24bdisposed within thespace13 wherebythermostat24 is operable to communicate signals to themicroprocessor30 to initiate operation of thefurnace10 under a so-called “call for heat” signal, such operation being well-known to those skilled in the art.
• For a particular furnace design and capacity for a three-stage furnace, for example, blower motor speeds for drivingblower22 sufficient to provide required pressures generated by the blower may be predetermined. Moreover, for various configurations of the furnace combustion products venting system, including the vent conduit orpipe25, for a particular furnace design, the blower speeds formotor23 may also be determined and which are sufficient to generate the required pressures. With respect to determining pressures, such pressures are normally measured as negative (below atmospheric pressure) in inches of water column. Hence, a high pressure is actually a greater amount of vacuum being pulled by theblower22 within theplenum14cor otherwise within the flowpath of ventilation air and combustion gases proceeding through theheat exchangers14. Since it has been determined there is a linear relationship between the required inducer blower speed for a predetermined amount of pressure generated by theblower22 and the length or configuration of the vent system, including the vent conduit orpipe25, a low furnace firing rate speed required of theblower22 for generating a low firing rate pressure may be determined and the low firing rate speed is related to the medium firing rate speed required of theblower22 for providing the required pressures at the medium firing rate. Moreover, if a learned medium firing rate inducer blower speed is obtained, then a relationship between the medium firing rate blower speed and the low firing rate blower speed may also be calculated, since it is a multiple of the learned medium firing rate blower speed. Accordingly, by basing the low firing rate inducer blower speed on a learned medium firing rate inducer blower speed which has been learned for a particular furnace installation, a multiplier may be applied to the learned medium speed value to determine the low firing rate speed of theblower22. Still further, since the position of thecontrol valve18 and the fuel gas pressure in manifold17 is correlated with the pressure produced by theblower22 within thefurnace10, unreasonably low manifold pressures which could create undesirable combustion characteristics are avoided.
• In addition to establishing a low firing rate speed ofblower22 and pressures within theheat exchangers14 produced thereby, the linear relationship between inducer blower speed and the configuration of the vent system, such as theconduit25, provides for determining the speed ofblower motor23 to produce suitable pressures in thefurnace10 commensurate with a high furnace firing rate and based on the learned medium firing rate blower speed. Moreover, the relationship between the required pressures generated by theblower22 for a particular firing rate, such as a medium firing rate, and the blower motor speed required to obtain such pressures, may be used to set the inducer blower speeds and attendant pressures for a continuously variable firing rate, based on a table of blower speeds versus vent system effective length for the vent system orconduit25. This data can be furnished from themodule35 and input to theprocessor30 for aparticular furnace10, as previously mentioned.
• One preferred method of setting the respective speeds for theinducer blower22 is indicated inFIG. 3. At “power-up” or start, when a call for heat is received atstep40,blower motor23 is set to operate at a predetermined medium firing rate default speed and a low speed (low firing rate) pressure switch position is checked atstep42. If the low firing rate pressure switch is not closed, if such a switch is being used, a fault signal is set bycontrol system26 atstep44. If the low firing rate pressure switch is closed, then the medium firing rate pressure switch is checked atstep46. If the medium firing rate pressure switch is not closed, theblower motor23 speed is incremented a predetermined minimum amount atstep48 and a status of the medium firing rate pressure switch is checked again atstep50.
• If, atstep50, the medium firing rate pressure switch is not closed, steps48 and50 are repeated until the switch is closed. If the medium firing rate pressure switch is closed atstep46, the process proceeds to step52 and the speed ofblower motor23 is decremented a minimum predetermined amount and the status of the medium firing rate pressure switch is checked again atstep54.Steps52 and54 are repeated until the medium firing rate pressure switch opens. Accordingly, within a relatively narrow range of pressure conditions for the furnace medium or intermediate firing rate, a suitable speed forblower22 is established and monitored by theprocessor30 via themotor control circuit23a.Once the medium firing rate blower motor speed forblower22 is established thecontrol system26 will query the database stored inmemories30aand/or30bto set the low firing rate blower speed forblower motor23 atstep56 and then the process may proceed to set the high firing rate speed forblower motor23 atstep58. Thefurnace10 then will continue to run atstep60 while blower motor speed forblower22 is monitored together with monitoring of the pressure switches32a,32band32c.
• Alternatively, thecontrol system26 may utilize a pressure sensor in place of plural pressure switches, which sensor continuously monitors pressures in a selected location or locations of the ventilating air and combustion gas flowpath throughheat exchangers14. The pressure settings at which action is taken may be carried out by thecontrol system26 by monitoring the pressure signal input from such a sensor to themicroprocessor30. For example, the medium firing rate speed ofblower22 could be set based on a limited range of suitable pressures for the medium firing rate. Blower speeds could be incremented or decremented from the aforementioned medium firing rate default speed until the pressure sensed by such a pressure sensor was within the predetermined range.
• Still further, the present invention contemplates that a single pressure switch may be used to set the medium firing rate blower motor speed forblower22 followed by the steps indicated inFIG. 3. In other words, steps42 and44 would be eliminated from the process shown inFIG. 3 and, at a call for heat, control over theblower22 would immediately proceed from the medium default speed to the medium learned speed based on the process ofFIG. 3 to establish a pressure within the furnace combustion system suitable for the specified firing rate, and the low and high firing rate blower speeds would then be determined in accordance with the process shown inFIG. 3.
• Those skilled in the art will recognize that an improved process and system for operating a multistage combustion furnace of the so-called inducer or ventilating type is provided by the present invention. Conventional engineering materials, components and procedures may be carried out to practice the invention. Although a preferred embodiment has been described in detail herein, those skilled in the art will also recognize that various substitutions and modifications may be made without departing from the scope and spirit of the appended claims.

Claims (17)

1. A method for operating a multistage combustion furnace, said combustion furnace including a heat exchanger including at least one combustion gas flowpath, an inducer blower for inducing the flow of air and combustion gases through said flowpath, for discharging combustion gases to a vent conduit, a blower motor drivably connected to said blower, pressure sensing means for sensing the pressure within the combustion gas flowpath at a predetermined location, a combustion fuel control valve and a control system including a microprocessor operably connected to said control valve, said blower motor, said pressure sensing means and to a thermostat for receiving signals for a call for heat for a space serviced by said furnace, said method comprising:

starting said combustion furnace at a first firing rate in response to a call for heat by said thermostat;

starting said blower motor at a predetermined speed for said first firing rate; and

setting a blower speed for said blower for a different firing rate based on the blower speed set for said first firing rate.

2. The method set forth inclaim 1 wherein:

said blower speed at said different firing rate is provided as a multiple of said blower speed for said first firing rate.

3. The method set forth inclaim 1 wherein:

said blower speed for said first firing rate is learned by setting a default speed for said first firing rate and selectively incrementing and decrementing said blower speed to provide a learned blower speed for said first firing rate.

4. The method set forth inclaim 3 wherein:

said blower speed for said different firing rate is set based on said learned blower speed.

5. The method set forth inclaim 1 including the step of:

setting a blower speed at second and third speeds for respective second and third firing rates based on the blower speed set at said first firing rate.

6. The method set forth inclaim 5 wherein:

said first firing rate is a medium firing rate of said furnace and said second blower speed corresponds to a low firing rate of said furnace.

7. The method set forth inclaim 5 including the step of:

said first firing rate is a medium firing rate and said third blower speed corresponds to a high firing rate of said furnace.

8. The method set forth inclaim 1 including the step of:

providing values of blower speed required to overcome combustion gas flow resistance of a venting system for said furnace to generate predetermined pressures in said furnace.

9. The method set forth inclaim 1 wherein:

said pressure sensing means comprises plural pressure sensing switches including at least a low firing rate pressure switch and a medium firing rate pressure switch; and

generating a fault signal for said furnace, if at furnace startup, said low firing rate pressure switch does not close.

10. A method for operating a multistage combustion furnace, said combustion furnace including a heat exchanger including at least one combustion gas flowpath, an inducer blower for inducing the flow of air and combustion gases through said flowpath, for discharging combustion gases to a vent conduit, a blower motor drivably connected to said blower, pressure sensing means for sensing the pressure within the combustion gas flowpath at a predetermined location, a combustion fuel control valve and a control system including a microprocessor operably connected to said control valve, said blower motor, said pressure sensing means and to a thermostat for receiving signals for a call for heat for a space serviced by said furnace, said method comprising:

starting said combustion furnace at an intermediate firing rate in response to a call for heat by said thermostat;

starting said blower at a predetermined speed for said intermediate firing rate; and

setting a blower speed for said blower for a different firing rate based on a multiplier applied to the blower speed set for said intermediate firing rate.

11. The method set forth inclaim 10 including the steps of:

selectively incrementing and decrementing said predetermined blower speed to provide a learned blower speed for said intermediate firing rate.

12. The method set forth inclaim 11 including the step of:

setting second and third blower speeds for respective low and high firing rates based on the learned blower speed set at said intermediate firing rate.

13. The method set forth inclaim 10 including the step of:

providing values of blower speed required to overcome combustion gas flow resistance of a venting system for said furnace to generate predetermined pressures in said furnace and basing said blower speed for said intermediate firing rate and said multipliers on said values of blower speeds required to overcome said flow resistance.

14. The method set forth inclaim 10 wherein:

said pressure sensing means comprises plural pressure sensing switches including at least a low firing rate pressure switch and an intermediate firing rate pressure switch; and

generating a fault signal for said furnace, if at furnace startup, said low firing rate pressure switch does not close.

15. A method for operating a three stage combustion furnace, said combustion furnace including a heat exchanger including at least one combustion gas flowpath, an inducer blower for inducing the flow of air and combustion gases through said flowpath, for discharging combustion gases to a vent conduit, a blower motor drivably connected to said blower, pressure sensing means for sensing the pressure within the combustion gas flowpath at a predetermined location, a combustion fuel control valve and a control system including a microprocessor operably connected to said control valve, said blower motor, said pressure sensing means and to a thermostat for receiving signals for a call for heat for a space serviced by said furnace, said method comprising:

starting said combustion furnace at a first firing rate in response to a call for heat by said thermostat;

operating said blower motor at a first blower speed for said first firing rate; and

setting second and third blower speeds for said blower for second and third firing rates based on said first blower speed.

16. The method set forth inclaim 15 including the steps of:

selectively incrementing and decrementing said first blower speed to provide a learned blower speed for said first firing rate prior to setting said second an third blower speeds.

17. The method set forth inclaim 15 including the step of:

providing values of blower speeds required to overcome combustion gas flow resistance of a venting system for said furnace to generate predetermined pressures in said furnace for said respective firing rates.

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